South Korea's Comprehensive Air-quality Index (CAI) Explained
South Korea's Comprehensive Air-quality Index — the CAI, also written CAI/통합대기환경지수 — is the number you'll see on AirKorea and in Korean weather apps. It runs 0 to 500 like the US AQI, but it collapses the range into just four grades, and it reports several pollutants in parts per million rather than µg/m³. This guide explains how to read it.
What the CAI is
The CAI summarises six pollutants: PM2.5, PM10, ozone (O₃), nitrogen dioxide (NO₂), carbon monoxide (CO), and sulphur dioxide (SO₂). Each gets a sub-index, and the reported CAI is the maximum — the same dominant-pollutant logic used elsewhere. Korea places particular emphasis on PM2.5 and PM10, which dominate the index during the spring "yellow dust" (hwangsa) episodes and winter accumulation.
The four grades
| CAI | Grade | Colour | In plain English |
|---|---|---|---|
| 0–50 | Good | Blue | A level that has no impact on daily activities. |
| 51–100 | Moderate | Green | Acceptable; sensitive people may feel mild effects on prolonged exertion. |
| 101–250 | Unhealthy | Orange | Sensitive groups should reduce outdoor activity; the general public should ease off strenuous outdoor exertion. |
| 251–500 | Very Unhealthy | Red | Everyone should avoid outdoor activity; sensitive groups should stay indoors. |
The sub-index breakpoints
PM2.5 and PM10 are reported in µg/m³ on a 24-hour average; the gaseous pollutants are reported in ppm, mostly on a 1-hour average.
| Grade | PM2.5 (24-hr, µg/m³) | PM10 (24-hr, µg/m³) | O₃ (1-hr, ppm) | NO₂ (1-hr, ppm) | CO (1-hr, ppm) | SO₂ (1-hr, ppm) |
|---|---|---|---|---|---|---|
| Good | 0–15 | 0–30 | 0–0.030 | 0–0.030 | 0–2.0 | 0–0.020 |
| Moderate | 16–35 | 31–80 | 0.031–0.090 | 0.031–0.060 | 2.1–9.0 | 0.021–0.050 |
| Unhealthy | 36–75 | 81–150 | 0.091–0.150 | 0.061–0.200 | 9.1–15 | 0.051–0.150 |
| Very Unhealthy | 76+ | 151+ | 0.151+ | 0.201+ | 15.1+ | 0.151+ |
How the CAI compares to the US AQI
The CAI's four grades are coarser than the US system's six categories, and its "Good" PM2.5 ceiling (15 µg/m³) is stricter than the US "Good" upper bound. A rough map:
- CAI 0–50 (Good) ≈ US AQI 0–50 (Good), but on a tighter PM2.5 cut
- CAI 51–100 (Moderate) ≈ US AQI 51–100 (Moderate)
- CAI 101–250 (Unhealthy) spans the US "Unhealthy for Sensitive Groups," "Unhealthy," and into "Very Unhealthy"
- CAI 251–500 (Very Unhealthy) ≈ US AQI 251+ (Very Unhealthy to Hazardous)
Because the single "Unhealthy" grade covers such a wide span (101–250), two days both labelled "Unhealthy" can be very different — it's worth checking the underlying PM2.5 number when the grade is orange.
Air quality on your home screen
Smog Report shows real-time air quality with widgets and Live Activities, and lets you switch between regional scales including South Korea's CAI. Free on iOS.
Download for iOSPrimary source: AirKorea — Introduction to the Comprehensive Air-quality Index